Box 3 on the 1099-MISC is "Other Income". It is normally taxable and should be reported on your tax return. If you e-file, then TurboTax will send a copy of the 1099-MISC to the IRS, so you don't have to.

Box 3 is used for a variety of things, like winnings from a contest. It is not considered self-employment income, so you will not have to file a Sch C (for this form anyway).

Since you have already filed your taxes, you will need to amend your return. 

The form for amending a return (1040X) is not ready yet, but since your return will need to be processed before amending and since the IRS did not start accepting e-files for processing until January 27th, you shouldn't amend your return for at least a few weeks.

Please watch this link (TurboTax FAQ on Amending Returns), and when the instructions for amending 2019 returns appear, you can start the amendment process then. Follow these instructions to open TurboTax and add your new W-2.

NOTE: this is a good reason to not e-file your return before the IRS opens up e-filing, because the time lag gives you a chance to make any corrections to your return before you actually file. Once you e-file the return, it is too late to change without amending the return instead.

NOTE NOTE: if by chance your e-file is rejected by the IRS, then you will have a chance to add your W-2 and then try to e-file again. Watch the status of your return.